Dear all,
I have some unwanted datasets to remove but I need their main histories, so in galaxy I just deleted the unwanted datasets but not the histories. Then I tried to purge the deleted datasets with the following commands: 

python scripts/cleanup_datasets/cleanup_datasets.py universe_wsgi.ini -d 0 -6 -r  
python scripts/cleanup_datasets/cleanup_datasets.py universe_wsgi.ini -d 0 -3 -r 

and I saw the this error:

/galaxy-dist/eggs/SQLAlchemy-0.7.9-py2.7-linux-x86_64-ucs4.egg/sqlalchemy/types.py:215: SAWarning: Dialect sqlite+pysqlite does *not* support Decimal objects natively, and SQLAlchemy must convert from floating point - rounding errors and other issues may occur. Please consider storing Decimal numbers as strings or integers on this platform for lossless storage.
Purged 0 datasets
Freed disk space:  0
Elapsed time:  0.0748069286346

Unfortunately I can't understand the warning and why nothing was deleted. Any help would be most appreciated.